A wideband wireless communication system is based on an orthogonal frequency division multiplexing (OFDM) scheme or an orthogonal frequency division multiple access (OFDMA) scheme, in which a physical channel signal is transmitted using a plurality of subcarriers so as to transmit data at a high speed.
In a wireless communication system, there is a need for synchronization between a transmitter and a receiver for data reception and demodulation. In particular, in a wireless communication system in which a channel environment between a base station (BS) and a mobile station (MS) is consistently changed, synchronization between the BS and the MS is performed through signaling, for successful data transmission and reception. Since the BS transmits a signal to several MSs at one reference timing in downlink, no problem occurs. However, since several MSs individually transmit signals in uplink and the signals reach the BS at different timings, propagation delay may be different. Accordingly, there is a separate timing synchronization method, which is referred to as ranging. That is, ranging enables several MSs to accurately adjust synchronization of a transmission time.
A plurality of MSs performs downlink synchronization with a data frame transmitted by a BS. As a method of enabling the MSs to perform synchronization, the BS may insert a preamble for synchronization into a part of a transmitted frame. Then, the MSs perform synchronization for a downlink channel through the preamble. Alternatively, the BS may use a separate synchronization channel.
In uplink, in order to avoid interference between the MSs and enable the BS to receive data, each MS transmits data to the BS through a time and/or frequency domain allocated thereto. Accordingly, for uplink synchronization, it is necessary to adjust synchronization between the BS and the MS through signaling in consideration of the channel environment of each MS.
An initial ranging process obtains an accurate timing offset between an MS and a BS and initially controls transmit power. If the MS is powered on, the MS acquires downlink synchronization from a received downlink preamble signal. Subsequently, the MS performs initial ranging in order to control an uplink timing offset and transmit power. Unlike initial ranging, periodic ranging periodically tracks an uplink timing offset and received signal strength after initial ranging. In addition, there are a bandwidth request ranging process of, at an MS, requesting a desired bandwidth from a BS and a handover ranging process performed by an MS for synchronization with another BS during handover.
Handover ranging is necessary to perform handover to a target BS at a moving MS. Handover ranging is equal to initial ranging except that the MS and the target BS may acquire a non-contention initial ranging transmission opportunity in order to perform a management message transmission/reception procedure for network entry/reentry. The MS may include information about a serving BS ID and a ranging purpose indicator in a ranging request message so as to inform the target BS that handover is currently being performed. In this case, if the target BS does not receive information about the MS in advance through a backbone, the target BS may request the information about the MS from the BS through the backbone.
It takes considerable time to receive all transmit power parameters associated with transmission of a ranging request message from the MS to the BS after initial ranging or handover ranging. Accordingly, there is a need for a new method for determining a power value used to transmit ranging request message by an MS.